The present invention relates to a starter used when an internal combustion engine is driven, and more particularly to a starter having a relatively large output.
It is a recent requirement of the starter of an internal combustion engine to be compact in size, light in weight, high in efficiency, and low in cost. To satisfy such requirements, it would be preferable to use a magnet type starter with a high speed-reduction mechanism. Furthermore, the motor itself must be compact. For example, the Japanese Patent Application Laid-open No. 10-115274 corresponding to U.S. Pat. No. 5,945,742 discloses this kind of starter for an internal combustion engine.
In general, a starter works as a motor. A magnet type starter satisfies the relationship V0=Vm+Vr, where V0 represents a power source voltage, Vr represents a voltage drop caused by a combined resistance r composed of a motor internal resistance, a battery internal resistance and a cable resistance between the battery and the motor, Vm represents a counter electromotive force appearing in the armature coil, and i represents a current flowing from the power source to the motor.
When D represents an outer diameter of the armature, lc represents a core width, n represents a rotational speed, and B represents a magnetic flux density of the magnetic field, the following relationship is established.Vm=k1·B·lc·D·n and 
Vr=i·r                 where k1 is a proportional constant (refer to FIG. 1).        
The above equations derive the following relationship.n=(V0−i·r)/(k1·B·lc·D) 
On the other hand, a torque T can be expressed by the following equation.T=k2·B·lc·D·i                 where k2 is a proportional constant, and a mechanical loss is, in this case, neglected.        
From the above equation, the output W can be expressed by the equationW=T·n=k·(lc·2·i·r·i2)                 where k is a proportional constant.        
For example, in case of a 12V battery, V0=12.
Accordingly, a maximum output value Wmax is given by the following equation (1) when the current value is i=6/r.Wmax=36·k/r  (1) 
When the equation (1) is evaluated in view of compactness of the motor, the factors D and lc which decide the motor dimensions have no relevancy to the output. Reducing these factors D and lc is effective in increasing the rotational speed n but decreases the torque T. However, according to a starter having an internal speed-reduction mechanism, changing the reduction ratio enables the starters to convert them into an arbitrary torque T and an arbitrary rotational speed n on the pinion output shaft. Thus, regarding the compactness, both the torque T and the rotational speed n have no adverse effects on the compact nature of the starters.
More specifically, the motor output increases with decreasing combined resistance r. The following equation (2) expresses the combined resistance r when the vehicle wiring resistance is neglected, wherein rB represents an internal resistance of the battery and rM represents an internal resistance of the motor.r=rB+rM  (2) 
As a result, reducing the motor internal resistance rM is a key to obtain a satisfactory output without increasing the size of a motor.
Furthermore, when evaluated with respect to the internal resistance of the starter, the magnet type starter is advantageous in having no series winding resistance.
However, the magnet type starter has the armature resistance rA and the brush contact resistance rT. Accordingly, the following equation (3) is established.rM=rA+rT  (3) 
The armature resistance rA has a direct relationship with the motor size (i.e. dimensions). When D represents the outer diameter of the armature and L represents an axial length of a coil constituting the armature (L=core width lc+both coil end lengths) (refer to FIG. 1), the following equation (4) is established.rA∝coil length/coil cross-sectional area rA∝L/D2 rA∝L/D  (4) 
The reason why the above-described conversion into the formula (4) is introduced can be explained in the following manner. When the outer diameter is small, the coil cross-sectional area is small too. A circumferential coil length and a radial coil length need to be reduced correspondingly. However, there is a limit in thinning the armature shaft. Thus, in order to secure a sufficient magnetic path area, the radial coil length cannot be decreased proportionally. Hence, it is realistic to replace D2 with D.
In any case, reducing the axial length L of the coil and increasing the outer diameter D of the armature is most effective in increasing the output (in other words, in reducing the armature resistance rA). This conclusion is applicable to a series winding type as well. Accordingly, in both of the magnet type and the series winding type, it is desirable to reduce the axial length of the coil and increase the outer diameter of the armature. In this case, the armature resistance rA represents a sum of an armature resistance and a field resistance. In short, to increase the output without increasing the size, it is necessary to reduce the value L/D.
FIG. 2 shows the relationship between an output of a conventional speed-reduction type starter and its combined resistance r. As the graph shows, actual measured data substantially agrees with the theoretical equation (1) shown by a solid line. The data separately shows different values of the internal battery resistance rB. In the drawing, classification of the data is based on the battery notation according to JIS. It is needless to say that a large output starter uses a battery having a small internal resistance rB.
The size of an internal resistance of a representative starter of 1.5 KW class is, for example, expressed by the following combination. In the case of a series winding type, the armature resistance is 2.0 mΩ, the field resistance is 2.0 mΩ, and the brush and other resistances is 1.0 mΩ As a result, a total resistance (i.e. the motor internal resistance rM) is 5.0 mΩ. In the case of a magnet type, the armature resistance is 2.0 mΩ, the field resistance is 0 ml, and the brush and other resistance is 3.0 mΩ. As a result, a total resistance (i.e. motor internal resistance rM) is 5.0 mΩ. In this case, the brush resistance is a value in a dynamic condition. The dynamic condition is a so-called dynamic frictional condition of the brush. To adjust the output, the magnet type uses a brush having a large internal resistance (i.e. having a small-amount of copper) so that the total resistance is equalized to 5.0 mΩ. In general, the series winding uses a carbon brush containing Cu by approximately 70%, while the magnet type uses a carbon brush containing Cu by approximately 60%.
In any case, approximately a half of the motor internal resistance rM is the armature resistance rA. Accordingly, the combined resistance r becomes small and the output increases with decreasing armature resistance rA. According to the formula (4), the armature resistance rA becomes small with increasing D or with decreasing L (or the ratio L/D). FIGS. 3, 4, and 5 show conventional examples.
The data shown in these drawings are classified into several groups according to the internal resistance of the battery. The relationship between rM and D is substantially theoretical. However, the relationship between rM and L or L/D is contrary to the theory. This is believed that, although L should be small to attain compactness and light weight, the heat capacity (D2L) of the starter should be enlarged to suppress the temperature rise when heat is generated. Increasing L is thus preferable in securing the heat resistivity. On the other hand, there may be a method for securing a sufficient heat capacity by increasing D. However, from the restriction such as the ring gear of an engine not being able to be enlarged, the pinion of a starter is then inevitably determined to a predetermined position and accordingly D cannot be enlarged. Furthermore, the circumferential speed of the brush will increase and accordingly a great amount of frictional heat will be produced. From these circumstances, the compactness and light weight must be sacrificed to assure a sufficient length L. As a result, no consideration about compactness is conventionally involved in optimizing the actual design relating to the dimension L/D. This tendency is remarkable when the battery scale becomes large because its current increases.
FIG. 6 shows the relationship between rM/rB and L/D to clearly (i.e. to exclude the influence of the battery) show the relationship between the battery scale and the starter resistance according to conventional starter data, in which rM/rB represents a resistance ratio of the internal resistance rM of the starter motor to the internal resistance rB of the battery.
Reducing the size ratio L/D in the motor design will result in increase of the output. However, as shown in FIG. 6, when the resistance ratio is small (i.e. when rM is small compared with rB), L/D tends to be large. This is contrary to the above-described motor characteristic theory. In short, the practical starters are compact (i.e. relatively small in the value L/D) when the resistance ratio is equal to or larger than 0.4 but are not compact (i.e. relatively large in the value L/D) when the resistance ratio is smaller than 0.4.
Furthermore, the starter output should be evaluated from various view points other than the size. FIG. 7 is a graph showing the relationship between the battery resistance and the starter resistance calculated according to the above-described equation (2). In this graph, solid lines represent the relationship between the starter internal resistance rM and the combined resistance r for various values of the battery internal resistance rB. Each black dot mark shown in the graph indicates a conventionally used combination of a starter and a battery. Furthermore, dotted lines show the relationship to the resistance ratio rM/rB. The ordinate axis shows the output obtained from the equation (1), too.
According to this graph, it will be understood that 1.8 KW is producible if an starter of rM=0.01 uses a battery having a small internal resistance (e.g. 12E). However, as indicated by black dot marks, it is a reality that the actual output is limited to 0.8 KW when combined with a battery of 12B from the consideration that the starter performance should be suppressed in view of heat resistivity.
On the other hand, a starter of rM=0.002 can produce 3.0 KW when combined with a battery of 12E and produce approximately 1.2 KW when combined with a battery of 12B. This is almost the same as an output producible when a starter of rM=0.01 uses a battery of 12C. In this case, as indicated by black dot marks, a starter is usually combined with a battery of 12E to produce 3.0 KW. Namely, from FIG. 7, it is apparent that the starter having the resistance ratio equal to or less than 0.4 (i.e. the starter to be improved in compactness) is the one producing a higher output equal to or larger than 2.0 KW. It will be understood that a high-output starter produces a great amount of heat and accordingly in the design of a starter the first priority is given to the improvement of heat resistivity.
In recent years, automotive vehicles must satisfy the severe requirements in improving their fuel economy. Furthermore, from the view point of reserving valuable global environments, an economy running control is recommended. Furthermore, there is the tenacious requirement for light weight, too. Especially, to carry out the economy running control efficiently, using a high-output starter at a derated output level is effective in attaining satisfactory performances in the durability, fuel economy, and startup operation. For example, according to this design concept, replacing a conventional starter of 1.2 KW with a new starter of 2.0 KW is considered to be advantageous in that a significant merit will be brought to the automotive vehicle as a whole. However, the engine room is very crowded and there is a limited amount of available space remaining in the engine room. Installing a large starter into such a limited narrow space is difficult. Hence, it is realistic to increase the starter output without increasing the starter dimensions. However, as described above, the high-output starters must be sufficiently heat resistant and it is generally difficult to reduce the body size. In short, the low-output starters of 1.0 KW class or less can optimize their motor design. On the other hand, the high-output starters of 2.0 KW class or above cannot employ an ideal (compact) motor design because of the requirement of suppressing heat generation.